Autophagy and Akt promote survival in glioma.

1Department of Neurology, Neurological Surgery and Brain Tumor Research Center, Helen Diller Family Comprehensive Cancer Center, University of California, San Francisco, CA, USA.

Abstract

Signaling through phosphatidylinositol 3-kinase (PtdIns3K)-Akt-mTOR is frequently activated in cancers including glioblastoma multiforme (GBM), where this kinase network regulates survival. It is thus surprising that inhibitors of these pathways induce minimal cell death in glioma. We showed that the dual PtdIns3K-mTOR inhibitor PI-103 induces autophagy in therapy-resistant, PTEN-mutant glioma, with blockade of mTOR complex 1 (mTORC1) and complex 2 (mTORC2) contributing independently to autophagy. Inhibition of autophagosome maturation synergizes with PI-103 to induce apoptosis through the Bax-dependent intrinsic mitochondrial pathway, indicating that PI-103 induces autophagy as a survival pathway in this setting. Not all inhibitors of PtdIns3K-Akt-mTOR signaling synergize with inhibitors of autophagy. The allosteric mTORC1 inhibitor rapamycin fails to induce apoptosis in conjunction with blockade of autophagy, due to feedback-activation of Akt. Apoptosis in the setting of rapamycin therapy requires concurrent inhibition of both autophagy and of PtdIns3K-Akt. Moreover, the clinical PtdIns3K-mTOR inhibitor NVP-BEZ235 cooperates with the clinical lysosomotropic autophagy inhibitor chloroquine to induce apoptosis in PTEN-mutant glioma xenografts in vivo, offering a therapeutic approach translatable to patients.

Autophagy and Akt promote survival in glioma. Induction of apoptosis in response to inhibition of PtdIns3K-Akt and mTOR kinase required inhibition of autophagy. Autophagy is a survival pathway in this setting, as a dual PtdIns3K-mTOR inhibitor induces apoptosis in Atg5 knockout, but not in Atg5 wild-type MEFs. Inhibition of PtdIns3K-Akt neither blocks mTOR nor induces autophagy, and therefore does not cause apoptosis in combination with inhibition of autophagy. Allosteric inhibition of mTORC1 does induce autophagy, but also activates Akt as a second survival signal. Unlike allosteric inhibitors of mTORC1, inhibition of mTOR (kinase) blocks both mTORC1 and mTORC2, induces apoptosis in conjunction with blockade of autophagy, and does not activate Akt. Importantly, 4-hydroxytamoxifen (4HT) treatment of glioma cells transduced with AKT-ER (an allele of Akt fused to the steroid binding domain of the estrogen receptor) leads to activation of Akt, and blocks apoptosis driven by inhibition of PtdIns3K, mTOR and autophagy. Thus, apoptosis in response to combining inhibitors of mTOR with inhibitors of autophagy also requires inhibition of Akt.